I. Field of the Invention
This invention relates to gas and liquid backflow-preventing devices for drains e.g. floor drains used in buildings to prevent flooding or to remove spillage. More particularly, the invention relates to devices of this type which permit liquid to pass into drains without difficulty, but prevent gases and water from escaping from the interior of the drains.
II. Description of the Prior Art
It is commonplace to provide drains of many kinds with U-shaped bends at or near their entrances in order to prevent escape of gases from the interior of the drains (such bends are often referred to as "P-traps"). The U-shaped bends retain a portion of the liquid passing through the drains and the liquid prevents the passage of gases from the drains to the exterior, or vice versa, but allows the passage of liquids in both directions.
Unfortunately, this form of gas sealing arrangement fails if a drain is used infrequently because the liquid trapped in the U-shaped bend dries out after a period of time, if it is not replenished, and then gases are free to escape from the drain. There are many instances in which drains are only infrequently used, so this problem can be quite widespread. For example, floor drains are frequently provided in the basements of homes to avoid flooding but such drains are rarely used and drying out of the P-trap is commonplace. This may not only result in the escape of unpleasant odours into the dwelling, but may also result in the release of dangerous gases such as radon or inflammable vapours.
This problem has been recognized in the past and various attempts have been made to provide alternative arrangements. For example, U.S. Pat. 4,744,109 issued May 17, 1988 to Grenville K. Yuill relates to a gas-sealing insert that fits into the entrance of a floor drain and makes use of a ball valve or movable plate to seal a small additional P-trap so that the insert remains closed against gases even if liquid in the trap dries out. The ball valve or hinged plate is located downstream of the trap so that it is automatically moved out of the way of liquids when the drain is in use. However, when the flow of liquid stops, the valve is biased by gravity or a spring to return to the sealing position. Inserts of this type have been very effective, but they have the disadvantage that the tubing used for the trap must be quite narrow because a U-shaped portion of the trap has to be positioned inside the mouth of the drain. This limits the flow of liquid the drain can accommodate and may result in backing up of the liquid upstream of the insert. Moreover, with inserts of this type, the ball valve or hinged plate is not very accessible, so if a solid article lodges in the device and impedes the movement of the valve, it is not easy to remove the article or even to see that the insert may not be functioning correctly.
To avoid these disadvantages, we have made attempts to eliminate the small additional P-trap and to position a movable gas-sealing valve closer to the entrance of the drain so that it is more visible and more easily accessible. However, if a downwardly movable spring loaded valve is provided at the entrance to the drain a short distance upstream of a conventional P-trap, we have found that an unexpected problem arises. If water enters the drain quite slowly, as is often the case, the water seeps around the edges of the valve without opening the valve sufficiently to allow gas to escape upwardly through the valve. Because the P-trap is rarely full to the maximum extent that results in water introduced on the upstream side causing displacement of an equal amount of water on the downstream side, water seepage through the valve starts to fill the drain tube leading from the valve to the P-trap and this compresses air present in this part of the drain tube. The resulting air pressure acts on the underside of the valve to seat the valve more firmly in the closed position, so that additional water flow through the valve is at some stage prevented. This causes water to collect above the valve until the depth of water creates a pressure on the valve that exceeds the air pressure beneath the valve and any spring force used to keep the valve closed. The valve is then opened, but the resulting flow of water through the valve may be erratic because the valve only opens as far as necessary and the compressed gas may not easily escape upwardly around the valve.
This unexpected problem has made such valve designs very unreliable and has undermined their commercial potential.
There is therefore a need for improved gas sealing devices for drains that function more advantageously.